The White House Situation Room put out calls to President Clinton's top national security aides at 6:30 yesterday morning, reporting what sounded like an implausible development: News reports from Belgrade said the Yugoslav government had agreed to NATO demands for a settlement in Kosovo.

The president and his senior advisers spent the next several hours calling officials in Europe, poring over wire reports and asking different versions of the same question: What's the catch?

It was not until early afternoon that White House officials said another possibility began -- slowly, tentatively, incredulously -- sinking in. Maybe there is no catch.

The watchword, voiced by Clinton and everyone on down, was caution. Yugoslav President Slobodan Milosevic, they said, has broken too many previous pledges for them to put faith in his latest professions about peace.

While these public statements of wariness seemed entirely genuine, they mingled with other sentiments voiced only privately by Clinton's team. There was astonishment at how quickly the endgame of the 72-day war in the Balkans had arrived. And there was vast relief at how Clinton had apparently survived yet another harrowing walk across the high wire.

"There's no point in pretending," said one senior White House official, acknowledging surprise at how suddenly Milosevic had apparently folded his hand. This official, like many colleagues, had guessed that "Milosevic was going to go part of the way" toward meeting NATO's demands -- then stop and see if he could divide the alliance.

This scenario, according to several senior officials interviewed yesterday, seemed as likely as not on Wednesday evening. Deputy Secretary of State Strobe Talbott that night offered a decidedly neutral assessment of the state of diplomatic activity in a phone call from Bonn to White House deputy national security adviser James B. Steinberg. Talbott, who was in touch with the mission led by Russian envoy Viktor Chernomyrdin and Finnish President Martti Ahtisaari, said Milosevic had tried to begin a negotiation about NATO's terms but was turned down by the delegation.

Clinton and his national security team went to bed uncertain what to expect next. When the morning news reported that the Serbian parliament had accepted NATO's terms, there was a hastily arranged phone call among national security adviser Samuel R. "Sandy" Berger, Secretary of State Madeleine K. Albright and Defense Secretary William S. Cohen. Berger called Clinton about 7:30 a.m.

Wariness predominated. Maj. Gen. Donald Kerrick, the number three official at the National Security Council, urged White House staff members at an 8 a.m. meeting not to leap to conclusions about peace being at hand. It was not until early afternoon that assessments brightened. Clinton spoke by phone with Talbott, who by this time had met in person with Ahtisaari, and received assurances that Milosevic had indeed pledged to accept the NATO terms.

The fact that the Yugoslav leader chose what at least appears to be capitulation was attributed by White House officials yesterday to two factors. One was what intelligence reports described as increasing disaffection among other senior officials in Belgrade about continuing the war.

The other factor was that NATO found the right diplomatic combination to appeal to Milosevic. Chernomyrdin's Russia has a history of strong sympathy toward Yugoslavia's Serb majority. Ahtisaari's role as a representative of the European Union allowed Milosevic to at least pretend that he was acquiescing to a body other than the despised NATO.

"He looked to his left and to his right, and saw that there was nowhere to go," said one senior policymaker, invoking a football metaphor: "There was no hole to run in between the center and the guard."

But Clinton's running room, likewise, was becoming cramped -- forcing a painful decision that yesterday's developments may let him avoid. In the face of Yugoslavia's seeming implacability, the president in recent days was in the midst of intensive deliberations about whether and how to abandon NATO's air-only strategy -- a move fraught with enormous diplomatic, military and political risks.

Even yesterday, he held a meeting with the military Joint Chiefs of Staff to talk about, among other things, various options for ground combat forces. The chiefs aired strong concerns at the meeting about ground action, a senior official said, but with less intensity than some aides had expected: "There was no edge in it." The meeting also focused on plans for a ground force that Clinton is more ready to use -- a peacekeeping force for Kosovo, which would enter with the consent of the Belgrade government.

A variety of senior officials said they expect it soon will be obvious whether the apparent accord is real or fraudulent. By the end of the weekend, sources said, it should be clear whether Yugoslav military and Serb special police forces are beginning their withdrawal from Kosovo; within a week, that withdrawal, and other agreements necessary for the arrival of the peacekeeping force, should be nearly complete.

On a fast-moving day, Clinton and his aides spent a good portion of their time deciding how he should respond publicly to fragmentary reports of encouraging moves by an unreliable adversary. Berger, sources said, was most cautious. The draft statement he offered for the president to read was so wary it sounded downright pessimistic. Other aides urged Clinton to show at least a glimmer of optimism. Just after 2 p.m., he read a statement to reporters in the Rose Garden, then quickly pivoted away from shouted questions.

Milosevic's apparent movement was "welcome," Clinton said. "But based on our past experience, we must also be cautious."

CAPTION: President Clinton prepares to make his Rose Garden statement, after which he did not take questions.

CAPTION: President Clinton said that until Belgrade carries out the accord, "we will continue to pursue diplomacy, but we will also continue the military effort."